A typical shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gear sets. A powertrain of such an automatic transmission that includes the plurality of planetary gear sets changes rotating speed and torque received from a torque converter of the automatic transmission, and accordingly changes and transmits the changed torque to an output shaft.
It is well known that when a transmission realizes a greater number of shift speeds, speed ratios of the transmission can be more optimally designed and therefore a vehicle can have better fuel mileage and better performance. For that reason, an automatic transmission that enables more shift speeds is under constant investigation.
In addition, with the same number of speeds, features of a powertrain such as durability, efficiency in power transmission, and size depend a lot on the layout of combined planetary gear sets. Therefore, designs for a combining structure of a powertrain are also under constant investigation.
A manual transmission that has too many speeds causes inconvenience of excessively frequent shifting operations to a driver. Therefore, the positive features of more shift-speeds are more important for automatic transmissions because an automatic transmission automatically controls shifting operations basically without needing manual operation.
In addition to various developments regarding four and five speed powertrains, powertrains of automatic transmissions realizing six forward speeds and one reverse speed have recently been introduced, an example of which is found in U.S. Pat. No. 6,071,208 that was issued on Jun. 6, 2000.
FIG. 15 illustrates a powertrain of the U.S. Pat. No. 6,071,208, and FIG. 16 shows an operational chart for the powertrain.
Referring to FIG. 15, the powertrain of the U.S. Pat. No. 6,071,208 includes a double pinion planetary gear set PG1 and a pair of single pinion planetary gear sets PG2 and PG3. A first carrier 4 is fixedly connected to an input shaft 2, and a second carrier 22 always acts as an output element.
Regarding connections between operational elements, a first ring gear 6 and a third ring gear 8, a second sun gear 12 and a third sun gear 10, and a second ring gear 16 and a third carrier 14 are fixedly interconnected, respectively. Meanwhile, the first carrier 4 is variably connected to a first sun gear 18 and the third carrier 14 interposing a first clutch C1 and a second clutch C2, respectively.
In addition, the powertrain further includes a first brake B1 that can stop rotation of the fixedly connected second and third sun gears 12 and 10, a second brake B2 that can stop rotation of the third carrier 14, a third brake B3 that can stop rotation of the first and third ring gears 6 and 8, and a fourth brake B4 that can stop the first sun gear 18.
As described above, the six-speed powertrain of U.S. Pat. No. 6,071,208 includes six friction elements of two clutches and four brakes. However, it is preferable to use fewer friction elements to enable six forward speeds and one reverse speed so that an automatic transmission can be more light and compact.
FIG. 16 is an operational chart for the powertrain of U.S. Pat. No. 6,071,208, and FIG. 17A-17F are charts showing operation states obtained when the powertrain is operated according to the operational chart in FIG. 16.
In particular, FIG. 17A shows detailed specifications of the powertrain of U.S. Pat. No. 6,071,208, i.e., gear ratios of each planetary gear set. FIG. 17B shows speed ratios in each shift-speed of the powertrain obtained by the detailed specification of FIG. 17A. In addition, FIG. 17C shows rotation speeds of each operational element relative to that of the input element, for each shift-speed. FIG. 17D shows slip speeds of friction elements at each shift-speed. FIG. 17E shows torque loads that each operational element or each friction element undertakes. FIG. 17F shows planetary gear sets that take part in power transmission in each shift-speed.
As shown in FIG. 16, the powertrain of U.S. Pat. No. 6,071,208 operates the first and fourth brakes B1 and B4 at a first speed, the first clutch C1 and the first brake B1 at a second speed, the second clutch C2 and the first brake B1 at a third speed, the first and second clutches C1 and C2 at a fourth speed, the second clutch C2 and the fourth brake B4 at a fifth speed, and the second clutch C2 and the third brake B3 at a sixth speed, respectively. The second and fourth brakes B2 and B4 are operated at a reverse speed.
Referring to the operational chart, the operation state of each operational element of the powertrain of U.S. Pat. No. 6,071,208 is described in detail. The planetary gear sets of the powertrain are supposed to have gear ratios shown in FIG. 17A such that the speed ratios shown in FIG. 17B are achieved.
(1) At the third forward speed, the first sun gear 18 rotates at a speed of more than twice that of the rotation speed of the input shaft (refer to FIG. 17C). In addition, the slip speed of the fourth brake B4, which is not operated in the third speed, becomes as high as that of the first sun gear 18 (refer to FIG. 17D).
The third forward speed is frequently engaged in the case that acceleration is needed, since a six-speed automatic transmission usually achieves the speed ratio of 1:1 at the fourth forward speed. Therefore, durability of an automatic transmission deteriorates if an element always rotates at a high speed in such a shift speed.
(2) Referring to FIG. 17D, slip speeds of friction elements are excessive for all speed ranges, which deteriorates durability of an automatic transmission and also causes excessive power loss. Therefore, the powertrain should be improved to have lesser slip speeds of friction elements for speeds D2-D6.
In particular, the sum of slip speeds of friction elements becomes excessively large at the sixth forward speed D6, and therefore, the durability problem is at its maximum at the sixth forward speed.
(3) Referring to FIG. 17F, when considering the number of planetary gear sets that take part in power transmission, at least two planetary gear sets take part in the power transmission for the fifth and sixth speeds, which deteriorates power efficiency. It is preferable that efficiency of power transmission is improved.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.